BPF-Regs - Using as

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9.7.2.2 Register Names

The eBPF processor provides ten general-purpose 64-bit registers, which are read-write, and a read-only frame pointer register:

‘%r0 .. %r9’: General-purpose registers.
‘%r10’: Frame pointer register.

Some registers have additional names, to reflect their role in the eBPF ABI:

‘%a’: This is ‘%r0’.
‘%ctx’: This is ‘%r6’.
‘%fp’: This is ‘%r10’.